Micropuncture experiments on distal convoluted tubules indicate that parathyroid hormone and cyclic AMP enhance calcium reabsorption in this nephron segment. Changes in colloid osmotic pressure in the vascular perfusate of isolated perfused rat kidneys led to parallel changes in absolute fluid reabsorption by pump perfused proximal tubular segments of such kidneys. Lowering of peritubular sodium concentration results in a reduced hydro-osmotic response of cortical collecting tubules. A similar maneuver in proximal convoluted tubules (isolated perfused proximal tubules) reduces net reabsorption of sodium and of water.